As a method of producing a silicon single crystal used for fabrication of a semiconductor device, CZ method wherein a single crystal is pulled from a melt in a quartz crucible with growing the single crystal has been widely conducted. In a CZ method, a seed crystal is immersed in a silicon melt in a quartz crucible in an atmosphere of inert gas, and then a predetermined silicon single crystal is grown by pulling the crystal with rotating the quartz crucible and the seed crystal.
In recent years, the demand to a silicon single crystal wafer has been severer with a tendency of a semiconductor device to be highly integrated and fine. Especially, influence of grown-in defects of a crystal in a device process has been pointed out recently, and various methods for improvement have been proposed.
Generally, in a silicon single crystal, there exist two kinds of internal (Intrinsic) point defects which are vacancies (Vacancy) and interstitial silicon atoms (Interstitial Si), those are grown to observable secondary defects by history after crystal growth. Grown-in defects due to vacancies are called FPD (Flow Pattern Defect), COP (Crystal Originated Particle), LSTD (Laser Scattering Tomography Defect) or the like depending on an evaluation method. The actual condition thereof is considered to be an octahedral void-type defect which is an agglomeration of vacancies (Analysis of side-wall structure of grown-in twin-type octahedral defects in Czochralski silicon, Jpn. J. Appl. Phys. Vol. 37 (1998) p-p. 1667-1670).
On the other hand, the defects due to interstitial atoms are formed as an agglomeration of interstitial silicons, and are called a dislocation cluster, LEP (Large Etch Pit), or the like (Evaluation of microdefects in as-groun silicon crystals, Mat.Res.Soc.Symp.Proc.Vol. 262 (1992) p-p. 51-56).
The octahedral void defect affects characteristics such as a gate oxide dielectric breakdown voltage (GOI: Gate Oxide Integrity) of a semiconductor device, and the dislocation cluster affects adversely characteristics such as junction leakage.
It is shown that an amount of introduced growth defects as above depends on a temperature gradient of a crystal at a growth interface and a growth rate of a silicon single crystal (The mechanism of swirl defects formation in silicon, Journal of Crystal growth, 1982, p-p 625-643). As for a method for manufacture of the silicon single crystal having few defects using the above fact, it has been disclosed that a growth rate of a silicon single crystal should be lowered in Japanese patent application Laid-Open (kokai) NO. 6-56588, and that the crystal should be pulled at a rate not exceeding the maximum pulling rate of the single crystal almost proportional to the temperature gradient of the boundary area between the solid phase and the liquid phase of the silicon single crystal in Japanese patent application Laid-Open (kokai) NO. 7-257991. There has also been reported an improved CZ method which takes note of a temperature gradient (G) and a growth rate (V) during growing of a crystal (Journal of the Japanese Association for crystal Growth, vol. 25, No. 5, 1998).
In addition, with regard to each kind of defects, there have been proposed various method for controlling octahedral void defects, and they can be roughly classified to the following two methods. One of them is a method wherein a defect density is reduced by controlling a thermal history of a crystal (slow cooling) (for example, publication of Japanese Patent Application No. 7-143391), and the other is that Vacancy is controlled by controlling a temperature gradient of a crystal and a pulling rate (for example, Japanese patent application Laid-Open (kokai) NO. 7-257991 described above). Furthermore, it has also been proposed that a dislocation cluster is controlled by controlling a temperature gradient of a crystal and a pulling rate (for example, Japanese patent application Laid-Open (kokai) NO. 8-330316).
However, the method currently shown here, for example as disclosed in Japanese patent application Laid-Open (kokai) NO. 8-330316, relates to a wafer grown at a low rate wherein OSF (Oxidation induced Stacking Faults, which is generated in a ring shape on a surface when the crystal is processed into a wafer) is disappeared at the center of the crystal, and it is a technology in which significant lowering of productivity and significant increase of cost cannot be avoided.
In this regard, it will be explained below in more detail. Based on the conventional technology in which OSF did not disappear at growth rate of 0.5 mm/min or more, the above technology is achieved by lowering a temperature gradient by means of a structure in a furnace which hardly cools a crystal to eliminate a difference of a temperature gradient in a crystal axis direction between a center part and a peripheral part, or to reduce a temperature gradient at a peripheral part of the crystal (that is, cooling from the peripheral part of the crystal is reduced, and thereby the temperature gradient in a crystal axis direction is necessarily reduced, and thus a growth rate is further lowered). Accordingly, in such a method, that is, in a conventional technique, it is necessary to pull at a lower growth rate than 0.5 mm/min at which an OSF ring region disappears at a center, and thus significant increase of cost cannot be avoided. Especially, high quality wafers having a diameter of 200 mm or more is used predominantly at present, and a method for mass-producing the crystals by increasing a pulling rate has been required.
On the other hand, as a method for reducing octahedral void defects in the whole plane of the wafer at higher growth rate, in the method disclosed in Japanese patent application Laid-Open (kokai) NO. 7-257991, an OSF generating region is vanished at the crystal center by increasing the temperature gradient in a crystal axis direction in a solid-liquid interface, and thereby speeding-up of crystal pulling rate can be achieved. However, since it is achieved by making cooling from the peripheral part of the crystal as large as possible, a temperature gradient at the peripheral part is significantly large compared with that at the center part. Accordingly, it can be easily assumed that dislocation clusters have been generated.
As described above, in order to produce the crystal in which both octahedral void defects and dislocation clusters disappear by conventional technique, there has been only a method with quite low productivity and high production cost wherein a rate of pulling the crystal is made lower than 0.5 mm/min, which cannot be put into practical use on commercial purpose. The low productivity is a large problem especially in mass-production of a crystal with a large diameter.
Furthermore, when using a gas flow-guide cylinder arranged around a silicon single crystal in such a production method, since it is necessary to increase a distance between the gas flow-guide cylinder and a surface of a melt to some extent in order to obtain-a uniform temperature gradient on a plane vertical to a growth axis of a single crystal, inert gas does not reach near the surface of the melt, and thus the evaporation effect of the oxygen atom from the surface of the melt by inert gas is reduced extremely. As a result, concentration of the interstitial oxygen in the produced silicon single crystal will become higher. Moreover, also in the case that the gas flow-guide cylinder is not used, the effect of evaporating oxygen atoms from the surface of the melt cannot be expected, and thus there is a problem that the silicon single crystal having a low concentration of interstitial oxygen cannot be obtained.
Moreover, if a silicon wafer manufactured from such a silicon single crystal having high interstitial oxygen concentration is subjected to heat treatment, for example, at high temperature of 1150° C. in an oxidizing atmosphere for about 2 hours, OSF may be formed as a defect induced by oxidation heat treatment in some cases. When the OSF is formed in a region where a device is to be fabricated, electrically serious failures such as leakage will be generated.
Furthermore, there is disclosed in Japanese patent application Laid-Open (kokai) NO. 11-79889 the silicon wafer with high quality wherein there is no generation of octahedral void-type defect which is an agglomeration of vacancies nor dislocation cluster-type defect formed as an agglomeration of interstitial silicons as a result of adjusting growth conditions. However, even in a region free from each of defects, there exist the region having extremely different precipitation characteristics in a plane of the wafer in the case that it is subjected to the heat treatment at 800° C. for 4 hours and at 1,000° C. for 16 hours in dry oxygen atmosphere as a precipitation heat treatment for interstitial oxygen. The precipitation characteristics of the interstitial oxygen is, of course, an important quality characteristics of a silicon wafer, since it has a close relation with a density of BMD (Bulk Micro Defect) formed during a fabrication process of a semiconductor device, and BMD becomes a gettering source for heavy metal impurities, or becomes a cause of a warp or slip of the wafer in the case that it is formed ununiformly.
As described above, since the rate of crystal growth is lowered in the case that the silicon single crystal with quite few growth defects is to be manufactured, it is not only inferior with respect to efficiency of industry, but there are problems in quality as for a wafer produced from this silicon single crystal that OSF is generated, and that a warp of the wafer and slip dislocations due to thermal strain may be generated by ununiform BMD density which is formed by precipitation of interstitial oxygen concentration in the plane of the wafer.